Apparatus and methods for investigation of radioactive sources in a sample

ABSTRACT

The invention provides an improved correction technique for use in analysing bodies of material containing radioactive sources. In particular the invention provides apparatus and a method, the method comprises a method of investigating radioactive sources in a body of material provided at an investigation location, the body of material comprising a plurality of samples, the method comprising detecting a portion of the emissions arising from a sample, the detested portion relating to a detected level, the detected level being corrected according to a correction method to give a corrected level, the method being repeated for one or more of the other samples, the correction method for one or more of the samples comprising providing a generator of radioactive emissions and detecting the radioactive emissions from the generator with the sample at investigating location, the relationship of the emissions detected with the sample at the investigating location to the emissions which would be detected with the sample absent from the investigating location determining a characteristic of the sample, the determined characteristic being employed as a factor in the correction method used for that sample to obtain the corrected level.

This invention is concerned with improvements in and relating toapparatus and methods for materials investigations. The invention isparticularly, but not exclusively, concerned with investigating gammaray emissions from materials. The invention is still more particularly,but not exclusively, concerned with correction techniques in suchinvestigations and the provision of a new correction technique.

In a variety of situations it is necessary to investigate emissions fromradioactive sources in or on materials to form a basis for a variety ofsubsequent decisions, actions or further considerations. Theinvestigations of the samples may relate directly to the emission, forinstance the emission source, or indirectly, for instance theconsideration of associated non-emitters or emitters which are notdirectly measurable. The emissions of interest are in particular gammaray emissions, but other emission forms may be considered additionallyor alternatively.

Emission investigation is particularly important in waste evaluationcases. For a given waste sample it is desirable to be able to determinea variety of unknowns. The unknowns may include, but are not limited to,one or more of the level, type, constituents, nature and distribution ofthe emissions, emission sources, associated materials or associatedfactors.

When taking measurements of the radioactive waste contained in a sampleusing prior art techniques it is necessary to make a number ofassumptions to gain a solvable system. One such assumption for certaincorrection techniques is that the materials are homogeneouslydistributed within a body or container and, in particular that thedensity profile of the material within the container is even. Given thevarious materials encountered and the varying size and shape of thematerials potentially making up the waste this is not a truly validassumption. Further variations occur in practice as materials settle ormove over time, for instance during transportation.

Relevant correction techniques accept this assumption and are qualifiedby an error level as a result. For safeguard reasons the level ofradioactive sources in a sample should be determined as accurately aspossible. In safety cases the error requires that the level be placed atthe worse case level. As a consequence the level of radioactive sourcesis frequently over estimated, with consequential complexity of furtherhandling or storage, as well as cost penalties.

The present invention seeks, amongst other aims, to provide a techniquein which greater account of variations or potential variations indensity are taken. The improved account may be used in accounting forthe effects of the material on the emissions as detected.

According to a first aspect of the invention we provide a method ofinvestigating radioactive sources in a body of material provided at aninvestigation location, the body of material comprising a plurality ofsamples, the method comprising detecting a portion of the emissionsarising from a sample, the detected portion relating to a detectedlevel, the detected level being corrected according to a correctionmethod to give a corrected level, the method being repeated for one ormore of the other samples, the correction method for one or more of thesamples comprising providing a generator of radioactive emissions anddetecting the radioactive emissions from the generator with the sampleat the investigating location, the relationship of the emissionsdetected with the sample at the investigating location to the emissionswhich would be detected with the sample absent from the investigatinglocation determining a characteristic of the sample, the determinedcharacteristic being employed as a factor in the correction method usedfor that sample to obtain the corrected level.

The body of material may be free standing, but is preferably containedin a container. The sample may include the part of the containerassociated with that part of the body of material.

Preferably the sample is a portion of the body of material which extendsfrom one side of the body through to the other. The sample may be asegment or slice through a body of material, preferably a segment orslice which extends outwards to the limits of the body of material intwo dimensions. Preferably the sample is taken horizontally through thebody of material. Preferably the sample has substantially the samethickness throughout the body of material. Preferably the thickness ofthe sample is its depth.

The samples may be investigated in order, for instance from one end ofthe body of material to the other.

Preferably the generator of emissions is a radioactive source providedexternally of the position occupied by the container and/or body ofmaterial in use. Preferably the generator is provided in opposition tothe detectors therefore.

The relationship of the emissions detected with the sample at theinvestigating location to the emissions which would be detected with thesample absent the investigating location may be the ratio of therespective count rates for the detectors. Preferably the emissionsleaving the generator count rate is determined in the absence of thesample. The absence may be an absence of any body of material in theinvestigating location. The relationship may be based on the ratioR/R_(o), where R is the rate at which the emissions are detected withthe sample in place and R_(o) is the rate of emissions which would bedetected without the sample in place. Preferably the relationship isbased on, and ideally equates to −ln(R/_(o)).

Preferably the characteristic determined is a function of the density ofthe sample and preferably the density of the sample. The characteristicmay be a function of the effective amount of material in the sample. Thecharacteristic may relate to the effective amount of material determinedto be in a sample relative to one or more other samples.

The determination of the characteristic for one or more or all of thesamples may be based on the interrelationship of a plurality ofpotential variables. Preferably the variables include one or more of,and most preferably all of:

I) the relationship of the emissions which would be detected without thesample present to the emissions detected with the sample in place, morepreferably the relationship of R and R_(o), where R is the rate at whichthe emissions are detected with the sample in place and R_(o) is therate of emissions which would be detected without the sample in place;

ii) the attenuation effects of the sample, more preferably μ, where μ isthe mass attenuation coefficient;

iii) the path of the emissions through the sample, more particularly, x,where x is the thickness of the sample (between source and detector);

iv) the density of the sample, ρ.

Preferably the determination of the characteristic is based on theequation:

R=R _(o) exp(−μρx)

where the symbols have the meanings referred to above.

Preferably under the conditions of the investigation x and/or μ, andmost preferably both are substantially constant. Preferably x is keptconstant by fixed relative generator, detector and sample positions (thesample may rotate without affecting this). Preferably μ is substantiallyconstant due to the energy of the generator emissions. An energy ofgreater than 400 keV, preferably greater than 1000 keV and ideallygreater than 1300 keV may be used. Preferably the source is as detailedin the British Nuclear Fuels PLC UK Patent Application no. 9900449.1filed 11 Jan. 1999 and referenced P17454, the contents of which areincorporated by reference. In particular we may provide a method ofinvestigating radioactive sources in a sample, the method comprisingdetecting a portion of the emissions arising from the sample, andfurther comprising the provision of a radioactive generator, passing atleast a portion of the emissions of the generator into the sample,detecting at least a portion of the emissions from the generator leavingthe sample, the radioactive generator emissions being of at least aplurality of emission energies and at least two of those energies beingdetected.

Preferably the method further provides that the detected portion of thesource emissions relate to a detected level for the sources in a sample,the detected level being corrected according to a correction method togive a corrected level for the sources in a sample, the process beingrepeated for one or more other samples.

Preferably the correction method employs measured transmissioncoefficients in determining the correction. The measured transmissioncoefficients, for one or more of the energies, most preferably all, maybe provided according to the equation:${{Trans}.\quad {Coeff}.} = \frac{R}{R_{o}}$

where R is the rate of detected photons with the sample in place, R_(o)is the rate of photons which would be detected without the sample inplace.

Preferably the density determined is used as a factor in the correctionmethod. The density used in the correction method may be an averageddensity from the determinations or a weighted average density from thedeterminations.

For correction of source emission energies corresponding to a generatorenergy preferably the measurement based correction factor is used. Forcorrection of source emission energies not corresponding to a generatorenergy preferably the correction factor is based on the extrapolation ofthe correction factors based on the measurements.

The generator is preferably a single isotope. Preferably the emissionenergies extend across a substantial portion of the range of energiesemitted from the sample. A substantial portion may be 50%, preferably75%, more preferably 90% and ideally 100% of the sample energies range.The generator most preferably of all emits energies encompassing therange of energies emitted by the sample. ¹⁵²Eu is a particularlypreferred generator. Preferably at least 5 energies from the source aredetected and used, more preferably at least 8 energies are detected andused.

Preferably the portion of generator emissions detected have passedthrough the sample. Preferably the generator is provided on the opposingside of the sample to the detectors, most preferably in directopposition.

One or more of the detectors for the sources may be used for detectingthe generator emissions and/or vice-versa.

We may also provide apparatus for investigating radioactive sources in asample, the apparatus comprising:

one or more detectors for emissions from the sources, the detectorsgenerating signals indicative of the emissions detected;

an investigating location into which the sample is introduced;

signal processing means for relating the detector signals to one or morecharacteristics of the sources;

a radioactive emission generator separate from the sample; and

one or more detectors for emissions from the radioactive generatorleaving the sample;

wherein the radioactive generator emissions are of at least a pluralityof energies and a least two of the plurality of energies are detected.

The source detectors and the generator detectors may be one and the samein the case of one or more or all of the detectors.

Preferably the amount of material in a sample is a function of theeffect on transmission of generator emissions by that sample, the totalamount of material being proportional to the effects of all the samples,the fraction of the total material in a particular sample being afunction, preferably a ratio, of that sample's effect on transmission tothe sum of all the effects. The effect on transmission of a sample maybe given a numerical value, the amount of material in that sample,and/or its mass, being defined as the numerical value for that sampledivided by the sum of the numerical values for all the samples,multiplied by the total mass. A density value for each sample may bemade in this way by virtue of the known volume of the sample.

Preferably the amount of material in a sample, Vs, is made proportionalto the ratio of R to R_(o), and more preferably is based on, and ideallyequates to: −ln(R/R_(o)). In this way the total amount of material inthe body of material is proportional to the sum of each sample amount,ΣVs. Preferably the fraction of the body of material in a given sampleis Vs/ΣVs.

Preferably the characteristic is determined based on a number ofvariables including one or more of, and ideally all of,:

I) the total mass of the body of material, M;

ii) the total volume of the body of material, V;

iii) the total number of samples forming the body of material, N;

iv) the amount of material in a given sample, Vs;

v) the sum of all the values proportional to the amounts of material inthe sample volumes, ΣVs;

vi) the density of the sample.

Preferably the characteristic is determined based on the equation:

ρ=(N.M/V)·(Vs/ΣVs)

Preferably the characteristic, and particularly the density is used incorrecting the detected level to the corrected level, for instance byestablished subsequent techniques, such as those set out in the LosAlamos primer, 2nd Edition, March 1991, ISBNO-16-032724-5.

According to a second aspect of the invention we provide apparatus forinvestigating radioactive sources in a body of material, the body ofmaterial comprising a plurality of samples, the apparatus comprising:

one or more detectors for emissions from the sources, the detectorsgenerating signals indicative of the emissions detected;

an investigating location into which the sample is introduced;

signal processing means for relating the detector signals to a detectedlevel for the sources;

processing means providing a correction method for correcting thedetected level for the sources to give a corrected level;

the apparatus further comprising:

a generator of radioactive emissions, at least of portion of theemissions entering the investigating location and, in use the sample;

one or more detectors for detecting the generator emissions with thesample at the investigating location;

processing means for determining a characteristic of the sample based onthe relationship of emissions detected with the sample at theinvestigating location to emissions which would be detected with thesample absent, the characteristic being employed by the processing meansas a factor in the correction method used for that sample to obtain thecorrected level.

The source emission detectors and the generator emission detectors maybe the same in one or more or all cases.

The signal processing means and/or processing means for the correctionmethod and/or processing means for the determined characteristic may beone and the same.

The second aspect of the invention may include any of the features,options and possibilities set out on the first aspect of the invention,including apparatus suitable for the implementation of the method stepsdetailed therein.

The first and/or second aspects of the invention may further include anyof the features, options, possibilities and steps set out below.

The sources may be singular or plural in disposition and/or type. Thesources may be one or more isotopes of one or more elements. The sourcesmay be alpha and/or beta and/or gamma emitters, but are preferably gammaemitters at least.

One or more sources of the same type and/or of different types may bepresent in the sample. The sources may be homogeneously distributed, ormore usually, unevenly distributed. The size and/or shape and/or mass ofa source may be different from the size and/or shape and/or mass ofanother source in the sample, be they of the same or different types.

The sources may be investigated by detecting one or more of theiremitting energies. Thus a characteristic energy of an isotope may bedetected.

The sources may be investigated directly, for instance they contributedirectly to the detected level, and/or the sources may be investigatedindirectly, for instance they do not contribute directly to the detectedlevel but are associated with sources which do.

The samples may be gaseous and/or liquid and/or solid. The samples maycontain one or more non-emitting or non-source materials. The materialsmay include one or more of metals, such as iron, steel, aluminum; wood;glass; plastics, such as Polythene, PVC; liquids, such as water.

The container preferably entirely encloses the body of material. Thecontainer may be of metal or of concrete or a combination of suchmaterials. Drums are a particularly preferred container, such as rightcylindrical drums.

The containers may be of one or more standard sizes. The height and/ordiameter of the containers may be standard.

Preferably the containers introduced to the investigating location oneat a time. The containers may be introduced by conveying along asurface, preferably a horizontal surface. The surface may include or beformed of a plurality of rollers. Preferably the container is removedfrom the investigating location in a manner equivalent to its,introduction.

The investigating location is preferably provided in proximity to theemission detector or detectors. The investigating location may beprovided in proximity to one or more radioactive sources. The sourcesare preferably intended to transmit radiation through the sample.Ideally the investigating location is provided between the detector(s)and the transmission source(s).

The sample, preferably the container for it, may be rotated at theinvestigating location. Preferably the rotation presents differentportions of the sample in proximity to the detectors and/or transmissionsource(s), the rotation may be continuous or stepped. The rotation maybe provided at between 5 and 25 rpm.

Preferably the sample and/or body of material and/or container areweighed at the investigating location, for instance by the turntableused to rotate it.

The sample, preferably the container for it, may be raised and/orlowered at the investigating location. Preferably the rasing and/orlowering presents different portions of the sample to the detector(s)and/or transmission source(s), the raising and/or lower may becontinuous or stepped. Preferably investigations are performed as thesample is lower and raised.

The sample may be rotated and/or lower and/or raised.

A single detector may be used. Preferably a plurality of detectors, forinstance three, may be used.

The detectors may be of the high purity germanium type.

Preferably the detectors are collimated to restrict their field of viewto the body of material of which the sample is the whole or a portionthereof. Where the sample is less than the whole of the body ofmaterial, preferably the detectors are collimated to restrict theirfield of view to the sample only. The sample is preferably a slice orsegment of the whole. The segments may be of the same thickness.

Preferably the detected level is obtained from a passive counting stage.Preferably the transmission based investigations are performed beforeand/or after the passive count stage.

Preferably the transmission source(s) is provided in opposition to thedetector(s). Preferably the same number of transmission sources areprovided as there are detectors. It is particularly preferred that thetransmission source be provided according to the nature of thetransmission source detailed in British Nuclear Fuels PLC UK PatentApplication no. 9900449.1 detailed reference P17454 filed on 11 Jan.1999 and as detailed above.

One or more surface dosimeters may be provided. Preferably the surfacedosimeters are configured to investigate gamma emitting sources. Alphaand/or beta emitting sources may alternatively or additionally beinvestigated.

Various embodiments of the invention will now be described, by way ofexample only, and with reference to the accompanying drawings in which:

FIG. 1 illustrates schematically an instrument suitable for implementingthe present invention;

FIG. 2 shows the density measurements obtained during scans of aninstrument employing the technique of the present invention, on fourconstructed, nominally, homogeneous material; and

FIG. 3 shows a density measurement obtained during a scan of aninstrument employing the technique of the present invention, on aconstructed non-homogeneous material.

The instrument illustrated in FIG. 1 is suitable for investigating gammaemission sources in a variety of situations and materials. The system 1is particularly designed to investigate waste samples presented in drums3 to investigating location 5.

The drums considered may be in a variety of sizes, but the instrument isreadily adapted to consider 100, 200 and 500 litre drums. Masses ofwaste, such as 40 kg to 550 kg, can readily be accommodated.

The drums 3 may be provided with a barcode which can be examined by abarcode reader on the system 1 so as to permanently assign resultsobtained to that drum 3.

The system 1 features a conveying table 7 formed of a large number ofparallel rollers 9, onto which the drums are lowered. Cranes, forklifttrucks and other lifting and manoeuvring means can be used to this end.

The conveying table 7 leads to the investigating location 5 and thenonward out the other side to a dismounting location 11 from which thedrums 3 are lifted. This set up allows the flow of samples forinvestigation through the system.

The drum 3 is supported at the investigating location 5 by a turntable13 which again is formed by a number of parallel rollers 15 mounted on amoveable frame 17. The frame 17 can be rotated about a vertical axis,using a motor (not shown), so as to rotate the drum 3 about itslongitudinal axis at the investigating location 5. A rotational rate of12 rpm is preferred, but stepped rotation can be used. The frame 17 canalso be provided with the option of being raised and lowered relative tothe level of the surrounding conveying table 7, using electrical drives,hydraulics or other systems (not shown), so as to adjust the verticalpositioning of the drum 3 within the investigating location 5. Thelowering and rotating motions may be applied simultaneously.

The turntable 13 and frame 17 are configured to weigh the drum 3 whilstit is on the turntable 13.

On either side of the investigating location 5 are investigatingassemblies 19, 21. In general one of these assemblies 19 acts as the,optional, transmission side and the other 21 as the detecting/receivingside for the investigations.

The detecting/receiving side 21 provides one or more detectors 23 forthe emission type under consideration. The detectors 23 are collimatedusing shields 25 to give a restricted field of view into theinvestigating location 5 for each detector 23. Through the use ofvariable aperture collimators (not shown) the range of radioactivitylevel for the waste which can successfully be handled is increased. Thefield of view is generally configured to be a slice through thatinvestigating location 5, the slice being substantially parallel to theturntable 13 and/or perpendicular to the axis of the drum 3 underconsideration.

For gamma rays the detectors may be of the germanium type, for instancehigh purity germanium detectors. LRGS or HRGS detectors may be used,with electrical or LN₂ cooling for HRGS detectors.

The provision of more than one detector, collimated to different fieldsof view, allows a greater number of measurements to be takensimultaneously, hence increasing the throughput for the system.

The detectors 23 monitor gamma rays originating in their field of viewin the drum 3 and in effect generate count rates. A count time of lessthan 30 minutes is generally employed.

The signals obtained from the detectors are fed to processingelectronics 27 and hence to a CPU 29 and operator display 31. Operatorcontrol and inputs are facilitated through-keyboard 33.

The processing electronics 27 are provided with error handling functionsand diagnostic facilities, as well as providing the appropriatecalibration functions.

Processing of the signals gives detailed information on the assay ofwaste material in the drum 3, the isotopic make-up of the waste. Moredetails of these analyses are discussed below. The results can be usedto classify waste according to the relevant disposal categories,including those below a deminimus level which can be characterized asnon-radioactive. The results can be expressed as the identification offission products, activation products or MGA code. The results can alsobe combined with the “fingerprinting” technique to give non-measurableisotope determinations.

The results obtained can be improved using a variety of potentialcorrection techniques. Correction based on weight and/or differentialpeak adsorption and/or use of a transmission source may be used. In thisinvention the new correction techniques detailed in the same applicant'sUK Patent Applications designated P17454 filed on 11 Jan. 1999 may alsobe used and details of those techniques are fully incorporated herein byreference. In particular to address certain problems the techniqueemploys a multi-energetic source as the transmission source for sampleinvestigation. The sources used are carefully selected to provideenergies spanning the important part of the spectrum for a number ofcommonly encountered waste types. The source material is exemplified by¹⁵²Eu. The intention is to provide a series of transmission basedinvestigations which bracket the emissions from within the sampleitself. Thus a more appropriate correction factor can be calculated forthe sample in question and its actual emission energies as the deviationof those energies from the energies at which the transmission effectsare actually measured and known are significantly reduced.

The overall effect of the multi-energy source is that the correction ismore accurate and the likely error is reduced.

The actual correction is obtained from the measured transmissioncoefficients, which at the respective energies are:${{Trans}.\quad {Coeff}.} = {\frac{R}{R_{o}} = {\exp \left( {{- {\mu\rho}}\quad x} \right)}}$

where R is the rate of detected photons, R_(o) is the rate of emittedphotons from the source, μ is the mass absorption: coefficient, ρ is thematrix density, x is the matrix thickness.

Measured transmission coefficients for two different samples withsignificantly different make-ups, at the various energies given by¹⁵²Eu, would give rise to significant variation with energy between thetwo samples, a variation which would not be apparent from a singleinvestigation at using transmission correction based on a single energy.

Between the greater number of actually measured values provided by thenew technique, an extrapolated value can be used. The greater number andrange of the measured values make this extrapolation more accurate too.

Furthermore, the other correction technique of GB 9900449.1 may be used.In this technique the material forming the sample is assumed to be madeup of three or more elements in unknown ratios. The transmissioncoefficient at energy i is the sum of the exponential terms for each ofthe constituents, i.e. for j terms. The definition is:$T_{nj} = {\exp\left( {- {\sum\limits_{j}^{\quad}\quad {q_{j}u_{ij}}}} \right)}$

where q_(j) is the effective matrix thickness for material j and μ_(ji)is the mass absorption coefficient for material j at energy i.

Three or more materials can be used in the determination, but it ispreferred that one low atomic number, one mid atomic number and one highatomic number constituent at least be used (i.e. low Z, mid Z and high Zelements). The elements may be hydrogen (low), aluminum (mid) and iron(high), for instance. It should be noted that the material selected neednot be a constituent of the sample for the technique to work.

Using this formula transmission coefficients can be calculated byvarying the q_(j) values. The variations are aimed at minimizing the sumof the residuals from a comparison of the measured and calculated/fittedcoefficients. The fitting may be a linear least squares approach (matrixsolution) or cycling through the possible q values. Once minimized a setof q_(j) values are reached which can be used to calculate accuratelythe transmission coefficient at any desired energy i.

In effect the technique assigns a fixed proportion of each compositematerial to best describe the unknown elemental composition of thesample. The result is an accurate transmission coefficient at any energyand hence full correction at any energy.

Through the use of a transmission source or sources 35 on transmissionside 19 in combination with stepped rotation of the drum 3 tomographicstyle investigations of the drum can be made to give plots of densitydistribution and radioactive distribution for the drum 3.

Surface originating alpha and/or beta emissions, and most preferablygamma emissions, for the drum 3 can also be measured using optionaldosimeters 37. The dosimeters are normally provided on the transmissionside 19 in proximity with the surface of the drum 3.

As discussed above three different known correction techniques (one ofwhich may be in the improved form detailed above) may be deployed. Theprinciples and operation behind each of these known forms is nowdiscussed.

Weight based correction seeks to account for the attenuating effects ofthe body of material the sources are in by a factor based on the body ofmaterials density. The total mass of the entire body of material isdivided by the total volume of the body of material, more commonly thetotal volume of the container for the body, to give an overall densityvalue for the entire body/container. This single value is then used inthe correction of the detected count to account for the reductionarising due to attenuation.

Differential peak absorption based correction again seeks to account forthe attenuation effects of the material, but through a more directinvestigation of attenuation. The gamma emissions from a sourceanticipated to be in the body of material at two characteristic energiesare considered. The ratio of the emissions at one energy to theemissions at the other energy is known (1:1, for instance, for Co-60)without attenuating materials, and this base ratio is compared with theactual ratio measured with the attenuating effects of the body ofmaterial present to give a factor relating to the attenuating effect andhence allowing correction.

Transmission source based correction comes in a variety of forms, buteach is generally based on determining attenuation effects on emissionsfrom within the body of material by measuring the attenuation onexternally sourced gamma emissions. Emissions at a known energy from thetransmission source are detected after their passage through the body.The ratio of the detected emissions with the body of material present iscompared with the detected emissions which would occur without thematerial. The attenuation is corrected for based on this difference. Theenergy of the transmission source is selected to be close to the energyof the emissions from the sample which need correcting.

Whilst the above mentioned correction techniques are all useful inaddressing a number of areas of potential error or variation in theresults, a number of assumptions are still applied in reaching thecalculated results from the measured information.

One of the particular areas of concern is the assumption that thedensity profile of the material in a container or drum underconsideration is even. That is to say the density of the top portion ofthe drum is the same as the middle of the drum and is the same as thebottom of the drum.

Clearly the material, such as waste, introduced into a container will inmany cases not be the same throughout the filling process. Even withinthe same general type of material variation occurs, for instance thesize of the pieces and hence how they fit together within the drum. Inmany case different materials will be introduced at different times asthe drum fills, potentially with those materials having differentdensities from one another. Additionally the materials may havedifferent properties, for instance fluids will tend to flow downward andoccupy the lower part of a drum fully, whilst potentially leaving voidsin the upper part of the drum. Still further problems occur in the caseof only partially filled drums where a substantial void exists at thetop of the drum as a result.

As a wide variety of variations in the density profile are likely to beencountered in practice, the prior art technique of weighing the drumand dividing that mass by the volume of the standard drum size inquestion, to give a single density value averaged for the whole drum isclearly a source of potential error in all cases and actual error inalmost all cases.

The correction technique of the present invention aims to combine weightmeasurements for the drum with transmission based measurements for thedrum in a meaningful manner to address the issue of accurate correction.

The technique is based on considering the contents of the drum as anumber of distinct parts, for instance horizontal segments through thedrum, each of the segments being treated separately from the others interms of its density value. By treating the drum as a number ofpotentially different density parts the errors referred to above aresubstantially removed. This division of the drum into segments andallocation of a density value to each segment cannot be made based onthe weight of the drum alone.

To achieve the aim the technique evaluates the amount of material in agiven segment by means of transmission source based investigations.

If R_(o) is the rate at which photons from an external source aredetected in the absence of the drum and R is the rate of detection ofthose photons with the drum present then:

R=R _(o) exp(−μρx)

where μ is the mass attenuation coefficient, ρ is the density of thesample and x is the thickness of the matrix.

For any given drum x will be constant as this is the path length throughthe drum (56 cm for a 200 litre drum, for instance). At high gammaenergies, say greater than 400 keV, μ can be treated as effectivelyconstant too. Experimental demonstration of this is provided below. Ineffect these valid assumptions make ρ proportional to −ln(R/R_(o)).

For a given segment or slice, s, through the drum, the amount ofmaterial in that slice, V_(s), is assigned the value −ln(R/R_(o)). Thetotal amount of the material present in the drum is thus defined asproportional to ΣV_(s); the sum being for all the slices of the drum.

Following this definition, the fraction of the whole body of material ina given slice, s₁, is given by V_(s1)/ΣV_(s). As the total mass of thedrum is known (from weighing), as the total volume of the drum is known(from measurement or as a standard size) and as the number of segmentsinto which the drum has been divided is known (from the analysiscontrol), then the density is defined as:

ρ(N.M/V)·(V _(s) /ΣV _(s))

The provision of more accurate density information specific to eachslice allows more accurate attenuation correction for that slice andhence greater overall accuracy.

The above mentioned technique was tested experimentally in two separatesets of experiments.

In the first set a series of homogeneous drums were scanned in positionon the turntable of the type of instrument described above in relationto FIG. 1. The scan was performed in 12 segments, in each case the drumwas presented on a carrying puck which in effect equated to the 11segment, with the 12 segment equating to the turntable itself and withthe first segment being above the top of the drum.

The results from this set of tests for a concrete, a hulls, a PVC and apaper carrying drum are set out in FIG. 2.

The results clearly show the change in density determined before thedrum is reached, segment 1, as the puck is reached, segment 10 and theturntable, segment 11.

More importantly the measurements reveal that the PVC drum is not ashomogeneous as thought.

Table 1 summarizes the density measurements derived from an assumptionof homogeneous material and those calculated by the transmission/weightcorrection based technique. Good agreement for all the densities ofmaterial was achieved.

TABLE 1 MATRIX KNOWN DENSITY TRANS/WEIGHT DENSITY Paper 0.066 +/− 0.0040.054 +/− 0.008 PVC 0.45 +/− 0.02 0.46 +/− 0.05 Hulls 1.47 +/− 0.07 1.49+/− 0.04 Concrete 2.10 +/− 0.10 2.12 +/− 0.08

The ability of the technique to pick up the variations in density isstill more apparent from the second set of tests. The results of thesetest are illustrated in FIG. 3 with regard to a deliberatelynon-homogeneous drum. The drum was constructed, working up from thebottom, from rubble (most dense), from paper and PVC (least dense) andwas topped off with bottles of water (intermediate density).

The figure clearly illustrates that the technique picks out each ofthese densities in turn.

The more accurate density measurements can be used to correct theprocessing of the results of the detectors to give a better resolutionof the level of radioactive material within the waste. The densitymeasurements could additionally or alternatively be used in the othercorrection techniques to improve them too.

When under taking a transmission/weight based correction of this typethe selection of an appropriate transmission source is important. Atransmission source of the type described in the British Nuclear FuelsUK Patent Application no. 9900449.1 filed on the same date as thisinitial application, under reference no. P17454 and detailed above, isparticularly suitable and the contents of that application areincorporated herein in that regard.

In general the source should be selected to have a highly penetratinggamma energy (for instance, in the range 1000 to 2000 keV) andpreferably be of high branching fraction. Use of such energy levelsources ensures that the assumption that u is constant remains a validone. The lack of variation of μ between materials is clearly illustratedin Table 2, at an energy of approx. 1500 keV.

TABLE 2 MATERIAL μ (g.cm⁻²) Iron 5.02E−2 Lead 5.27E−2 Magnesium 5.29E−2Sand 5.35E−2 PVC 5.49E−2 Water 5.94E−2 Polythene 6.11E−2 AVERAGE 5.50E−2

The penetrating power of the transmission source is also important asthis is a limiting factor on the density range which can be accommodatedby the present technique.

Table 3 sets out the fraction of photons which will penetrate a 56 cmdiameter drum, with μ=5.25E−2 g.cm⁻², for a variety of materialdensities between 0 and 3.0 g.cm⁻³. The count rate for the detector andassociated precision for a 15 minute each way assay with 12 segments(i.e. 75 seconds per segment) is shown too.

1 SIGMA ERROR DENSITY FRACTION COUNT COUNTS IN ON 75s (g.cm⁻³)TRANSMITTED RATE 75 SEC. COUNT (%) 0.0 1.0 893 67000 0.4 0.5 0.23 20515400 0.8 1.0 0.053 47.3 3550 1.7 1.5 0.012 10.7 800 3.5 2.0 0.0028 2.50188 7.3 2.5 0.00064 0.57 43 15 3.0 0.00015 0.13 10 32

Based on this approx. 1500 keV source densities up to 2.5 g.cm ⁻³ (abovethe upper level of most samples encountered in practice) could besuccessfully addressed. Increased penetrating power and increased counttimes could raise the density level still further.

As demonstrated above the technique offers improved performance andaccuracy of results, whilst being applicable to a wide variety of wastetypes, including partially filled drums and the hardest wastes normallyencountered, those of high density.

What is claimed is:
 1. A method of investigating radioactive sources ina body of material provided at an investigation location, the body ofmaterial comprising a plurality of samples, the method comprisingdetecting a portion of the emissions arising from a sample, the detectedportion relating to a detected level, the detected level being correctedaccording to a correction method to give a corrected level, the methodbeing repeated for one or more of the other samples, the correctionmethod for one or more of the samples comprising providing a generatorof radioactive emissions and detecting the radioactive emissions fromthe generator with the sample at the investigating location, therelationship of the emissions detected with the sample at theinvestigating location to the emissions which would be detected with thesample absent from the investigating location determining acharacteristic of the sample, the determined characteristic being afunction of the density of the sample, the determined characteristicbeing employed as a factor in the correction method used for that sampleto obtain the corrected level.
 2. A method according to claim 1 in whichthe relationship of the emissions detected with the sample at theinvestigating location to the emissions which would be detected with thesample absent the investigating location is the ratio of the respectivecount rates for the detectors.
 3. A method according to claim 1 in whichthe relationship equates to −ln(R/R_(o)), where R is the rate at whichthe emissions are detected with the sample in place and R_(o) is therate of emissions which would be detected without the sample in place.4. A method according to claim 1 in which the characteristic is afunction of the effective amount of material in the sample relative toone or more other samples.
 5. A method according to claim 1 in which thecharacteristic is a function of the effective amount of material in asample and the amount of material in a sample is a function of theeffect on transmission of generator emissions by that sample, the totalamount of material being proportional to the effects of all the samples,the fraction of the total material in a particular sample being afunction of that sample's effect on transmission to the sum of all theeffects.
 6. A method according to claim 5 in which the fraction of thetotal material in a particular sample is a ratio of that sample's effecton the transmission to the sum of all the effects of the samples on thetransmission.
 7. A method according to claim 5 in which the amount ofmaterial in a sample, Vs, equates to: −ln(R/R_(o)).
 8. A methodaccording to claim 7 in which the total amount of material in the bodyof material is proportional to the sum of each sample amount, ΣVs.
 9. Amethod according to claim 7 in which the fraction of the body ofmaterial in a given sample is Vs/ΣVs.
 10. A method according to claim 1in which the characteristic is determined based on a number of variablesincluding one or more of, and ideally all of: i) the total mass of thebody of material, M; ii) the total volume of the body of material, V;iii) the total number of samples forming the body of material, N; iv)the amount of material in a given sample, Vs; v) the sum of all thevalues proportional to the amounts of material in the sample volumes,ΣVs; vi) the density of the sample.
 11. A method according to claim 1where the characteristic is determined based on the equation:ρ=(N×M/V)×(Vs/ΣVs).
 12. A method of investigating radioactive sources ina body of material provided at an investigation location, the body ofmaterial comprising a plurality of samples, the method comprisingdetecting a portion of the emissions arising from a sample, the detectedportion relating to a detected level, the detected level being correctedaccording to a correction method to give a corrected level, the methodbeing repeated for one or more of the other samples, the correctionmethod for one or more of the samples comprising providing a generatorof radioactive emissions and detecting the radioactive emissions fromthe generator with the sample at the investigating location, therelationship of the emissions detected with the sample at theinvestigating location to the emissions which would be detected with thesample absent from the investigating location determining acharacteristic of the sample, the determined characteristic beingemployed as a factor in the correction method used for that sample toobtain the corrected level, the determined characteristic being based onthe equation: R=R _(o) exp(−μρx) where R is the rate at which theemissions are detected with the sample in place and R_(o) is the rate ofemissions which would be detected without the sample in place; μ is themass attenuation coefficient; x is the thickness of the sample (betweensource and detector); and ρ is the density of the sample.
 13. A methodaccording to claim 12 in which under the conditions of the investigationx and μ are substantially constant.
 14. A method according to claim 12in which x is kept constant by fixed relative generator, detector andsample positions.
 15. A method according to claims 12 in which μ issubstantially constant due to the energy of the generator emissions. 16.Apparatus for investigating radioactive sources in a body of material,the body of material comprising a plurality of samples, the apparatuscomprising: one or more detectors for detecting emissions from thesources, the detectors generating signals indicative of the emissionsdetected; an investigating location into which the sample is introduced;signal processing means for relating the detector signals to a detectedlevel for the sources; processing means providing a correction methodfor correcting the detected level for the sources to give a correctedlevel; the apparatus further comprising: a generator of radioactiveemissions, at least a portion of the emissions entering theinvestigating location and, in uses, the sample; one or more detectorsfor detecting the generator emissions with the sample at theinvestigating location; processing means for determining acharacteristic of the sample based on the relationship of emissionsdetected with the sample at the investigating location to emissionswhich would be detected with the sample absent, the determinedcharacteristic being a function of the density of the sample, thecharacteristic being employed by the processing means as a factor in thecorrection method used for that sample to obtain the corrected level.17. A method of investigating radioactive sources in a body of materialprovided at an investigation location, the body of material comprising aplurality of samples, the samples each being a slice which extends fromone side of the body of material to the other side of the body ofmaterial, the method comprising; detecting a portion of the emissionsarising from a sample using a detector, the detected portion relating toa detected level, the detected level being corrected according to acorrection method to give a corrected level, the method being repeatedfor the other samples forming the body of material; and the correctionmethod for one or more of the samples comprising providing a generatorof radioactive emissions of energy greater than 400 keV, detecting theradioactive emissions from the generator with the sample at theinvestigation location, detecting the radioactive emissions from thegenerator with the sample absent from the investigation location, therelationship of the emissions detected with the sample at theinvestigation location to the emissions detected with the sample absentfrom the investigation location determining a characteristic of thesample, the relationship being based on the ratio of that count rate foremissions detected with the sample at the investigation location to thatcount rate for emissions detected with the sample absent from theinvestigation location, the determined characteristic being a functionof the sample, the determined characteristic being employed as a factorin the correction method used for that sample to obtain the correctedlevel, the relative position of the generator, investigation locationand detector being fixed during the method.